Determination of mass concentration of air-dispersed particulate matter having definite granulometric characteristics, i.e., of the so-called PMx (“Particulate Matter x”, denoting atmosphere-suspended—aerosol—particulate matter having an <x μm aerodynamic diameter), represents a fundamental need to the ends of an objective evaluation of air quality conditions. Moreover, such a determination is mandated by regulations in force at international level.
The reference methodology related, e.g., to the determination of mass concentration of the particulate matter PM10 is based on the drawing of representative particulate matter samples, their collection on a filter means and the subsequent determination of their mass by a differential gravimetric technique referred to as “double weighing” (determination of the mass of a filter means before and after sampling).
However, when extensively applied to the ends of air quality control such a reference methodology entails two fundamental limitations, i.e.:                the operating complexity, requiring employ of skilled personnel over relatively long times and use of high-cost logistics and specific equipment, like, e.g., air-conditioned rooms,        the delay in informing the public about air quality conditions (a primary need stressed in international regulations)        
Given the limitations associated to gravimetric reference methodology, on networks in charge of air quality control automatic instrumentations are used, capable of providing, practically in real time, an estimate of the average mass concentration of particulate matter in the time period taken into account (typically, average concentrations every 24 h). However, performances of most automatic instrumentations are deemed unsatisfactory in the state of the art, as affording bias-affected mass concentration data, i.e., data affected by errors, often quantitatively, relevant ones.
Among automatic instrumentations, particularly relevant are those that for determining the mass concentration of particulate matter samples accumulated on filter means use the so-called β attenuation technique, based on the measurement of the attenuation of a flow of beta radiations (emitted, e.g., by a 14C source) traversing a homogeneous matter film.
Analytical components of the sampling and measurement process for the particulate matter are schematically depicted in the diagram of FIG. 1. Therefrom, it is highlighted that the overall sampling and measurement process is based on several subsequent phases. In general, for a strict analysis it is necessary to single out the causes of deviation from expected values in each phase and quantitatively estimate such deviations:Y=XPM+δT+δLa+δLc+δM+ε, wherein:                Y denotes the expected value of the measurement,        XPM represents the expected PMx value,        δT represents the deviation from theoretical granulometric cut efficiency,        δLa represents the presence of artifacts in the particulate matter accumulation phase,        δLc represents the presence of biases in the sample conditioning phase before measurement,        δM represents the mass measurement accuracy, and        ε represents the residual random error.        
As it is known, in mass, measurement techniques based on the β attenuation principle there are three basic error sources reducing measurement accuracy and reproducibility, i.e.:                intrinsic biases of the method (intrinsic accuracy);        systematic biases due to the implementation of the measurement technique (measurement reproducibility); and        random biases (random uncertainty), associated to the overall contribution of random-type deviations, among which those intrinsically linked to Poisson statistical distribution governing beta emission, those linked to the variations in the geometric repositioning of the filter matrix with respect to source and detector, etc.        
Concerning the intrinsic accuracy of the beta measurement technique (intrinsic biases of the method) there has to be mentioned that it is based on the laws governing beta radiation interaction with the charges constituting the traversed matter. Accordingly, determination of the mass of a definite film interposed between source and detector requires a careful evaluation of the functional dependence of the energy spectrum of incident beta electrons with respect to mass thickness of the film of material traversed, in proportion to the atomic number and the mass number of the film of measured material, the maximum value of the energy of incident electrons, as well as the geometry of the source-detector system, the lack of homogeneity of the filter substrate, the lack of homogeneity in particulate matter settling and the calibration procedures and techniques (adequate selection of the material of which the calibration foils are made, accurate calibration procedure), etc.
Among the most relevant sources of systematic biases undermining the reproducibility level of the measurement, the following need mentioning:                the counting efficiency of the real detector (e.g., Geiger-Muller) which depends on a set of parameters, among which the incident electron energy, the value of the power supply voltage, the response dead time, the efficiency of the signal processing and control chain, etc.;        the variations in density of the air present between source and detector, which may vary depending on environmental conditions (pressure, temperature, relative humidity, etc.);        the variations in the mass of hygroscopic filter media, and therefore of the mass thickness xf associated thereto, owing to exchanges of water vapor molecules with air present in the measurement chamber or with external air drawn during the sampling phase;        the mechanical repositioning of the filter medium (sample foil) between source and detector;        the presence of radionuclides in the particulate matter accumulated on the filter medium.        